This invention relates to spraying coatings, and particularly to the spraying of a coating of uniform thickness onto a circular area of a substrate.
Spraying of a coating of uniform thickness onto a disk or other circular area of a substrate presents unusual difficulties, particularly if the area has concentrically contoured elevations instead of being flat. Spraying of a flat surface is relatively easy and common, being effected by linear passes of overlapping spray stripes. Spray coating of the outer surface of a shaft is similarly done by slowly moving the spray stream lengthwise along a spinning shaft.
However, spraying onto a spinning disk ordinarily results in nonuniformity. If the spray stream is simply passed at constant speed over the spinning disk through the center, the coating will be much thicker at the center because the surface speed of the disk is slower there, being zero speed at the very center. The nonuniformity may be reduced by accelerating the movement of the stream from the edge toward the center, and decelerating from the center out. Very high speed, theoretically approaching infinite, is necessary but not very practical. The passes ay be made slightly off-center, but the problem still is not solved, partly because spray gun manipulators such as robots are designed to operate in steps and are not generally capable of smooth accelerations and decelerations. Therefore, there is a need for a better method of making passes of a spray stream over a spinning disk.
The need for spraying such surfaces particularly relates to the top domes of pistons for internal combustion engines. Advanced diesel engines are incorporating pistons with ceramic coatings for running hotter and enhanced performance. These coatings are being produced with the thermal spray process.
Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A conventional thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns. The material alternatively may be fed into a heating zone in the form of a wire. A thermal spray gun normally utilizes a combustion flame, an arc plasma stream or an electrical arc to produce the heat for melting of the powder particles.